ON THE TEMPERATURE OF THE FRESH- WATER LOCHS OF SCOTLAND. 417 



ordinary distribution of the isotherms in early spring. This diagram is from observa- 

 tions made by Sir John Murray in April 1887.* It shows that the isotherms are 

 very nearly vertical, and it was always found that, when the differences of temperature 

 throughout the loch were not great, the slopes of the isotherms were much greater than 

 when the differences of temperature were considerable. All efforts to trace the return 

 current directly and quantitatively have failed, though indirect indications of it are not 

 infrequent. In spring it should not be so well marked as in autumn, as in spring there 

 are no great differences of temperature and no well-marked condition of stratification. 

 The fact that the heating takes place so uniformly down to, say, a depth of 250 feet, can, 

 I think, only be explained on the assumption that in spring the influence of the return 

 current is felt to at least this depth. In autumn, however, it is the water which is being 

 cooled off which is blown along the surface, and also, at that time of year the loch is in 

 a well-marked stratified condition. The water which is blown along by the wind will 



Lochjfess zs* 1 *** -26?yfprCLlg87.s. w.cule. 



600 ' 



Fig. 5. 



accordingly have a tendency to sink. We know that at this time of year there is a 

 layer of water at the surface of nearly uniform temperature, which is originally due to 

 the mixing action of the wind in producing waves, and the action of the wind in blowing 

 water along the loch will have the further effect of increasing the depth of this layer. 

 There will also be a tendency towards the formation of this layer when at any time the 

 temperature of the air is lower than that of the water, so that even in midsummer 

 during the cool evenings and nights this influence is at work ; and this will account 

 for the early appearance of the Sprung schicht. As the Sprung schicht becomes more 

 marked the effect of the wind is almost entirely confined to this layer of approximately 

 uniform temperature, and the return current takes place mainly along the surface of 

 separation between the cold and warm layers. It is not easy to say how far the 

 influence of the return current is felt, but it would appear that it is not felt at any 

 great distance from the ends of the loch. Former deductions have been based on 



* Scottish Geographical Magazine, July 1888. 



